When the sun is in the active phase of its 11-year cycle, it can unleash powerful magnetic storms that disable satellites, threaten astronaut safety, and even disrupt communication systems on Earth.

The worst storms can knock out power grids by inducing currents that melt transformers.

Modern power grids are so interconnected that a big space storm — the type expected to occur about once a century — could cause a cascade of failures that would sweep across the United States, cutting power to 130 million people or more in this country alone, the new report concludes.

Such widespread power outages, though expected to be a rare possibility, would affect other vital systems.

"Impacts would be felt on interdependent infrastructures with, for example, potable water distribution affected within several hours; perishable foods and medications lost in 12-24 hours; immediate or eventual loss of heating/air conditioning, sewage disposal, phone service, transportation, fuel resupply and so on," the report states.

Outages could take months to fix, the researchers say. Banks might close, and trade with other countries might halt.

"Emergency services would be strained, and command and control might be lost," write the researchers, led by Daniel Baker, director of the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

"Whether it is terrestrial catastrophes or extreme space weather incidents, the results can be devastating to modern societies that depend in a myriad of ways on advanced technological systems," Baker said in a statement released with the report.

Stormy past

Solar storms have had significant effects in modern time:

— In 1989, the sun unleashed a tempest that knocked out power to all of Quebec, Canada.

— A remarkable 2003 rampage included 10 major solar flares over a two-week period, knocking out two Earth-orbiting satellites and crippling an instrument aboard a Mars orbiter.

"Obviously, the sun is Earth's life blood," said Richard Fisher, director of the Heliophysics division at NASA. "To mitigate possible public safety issues, it is vital that we better understand extreme space weather events caused by the sun's activity."

"Space weather can produce solar storm electromagnetic fields that induce extreme currents in wires, disrupting power lines, causing wide-spread blackouts and affecting communication cables that support the Internet," the report states. "Severe space weather also produces solar energetic particles and the dislocation of the Earth's radiation belts, which can damage satellites used for commercial communications, global positioning and weather forecasting."

While the sun is in a lull now, activity can flare up at any moment, and severe space weather — how severe, nobody knows — will ramp up a year or two before the peak.

Some scientists expect the next peak to bring more severe events than other recent peaks.

"A catastrophic failure of commercial and government infrastructure in space and on the ground can be mitigated through raising public awareness, improving vulnerable infrastructure and developing advanced forecasting capabilities," the report states. "Without preventive actions or plans, the trend of increased dependency on modern space-weather sensitive assets could make society more vulnerable in the future."

The report was commissioned and funded by NASA. Experts from around the world in industry, government and academia participated. It was released this week.